Microwave load isolators



Aug. 14, 1962 R. A. KRoGH MICROWAVE LOAD ISOLATORS Original Filed Sept. 13, 1955 2 Sheets-Sheet 1 INVENTOR.

Aug. 14, 1962 R. A. KRoGH 3,049530 MICROWAVE LOAD ISOLATORS Original Filed Sept. 13, 1955 2 Sheets-Sheet 2 IN V EN TOR.

United States Patent Ofiice 3,M9,680 Patented Aug. 14, 1962 3,049,680 MICROWAVE LOAD ISOLATORS Ray A. Irogh, Mountain Lakes, NJ., assignor, by mesne assignments, to Litton Industries, Inc., Beverly Hills, Calif., a Corporation of Delaware Continuation of application Ser. No. 534,072, Sept. 13, 1955. This application Dec. 28, 1959, Ser. No. 465 3 Claims. (Cl. 333-24) This invention relates to microwave load isolators, and more particularly to microwave load isolators of minimal length and weight Wherein a unitary element functions as an electrical connector, magnet mounting bracket, and constitutes a portion of a magnet-protecting shield.

Basically a microwave load isolator utilizes one or more ferrite specimens of predetermined size and shape and mounted in a waveguide to achieve one-Way transmission of microwave energy. More specifically, it has been found that by biasing the ferrite specimens With a magnetic field of predetermined magnitude the waveguide may be made to present a relatively low loss path in one direction and a relatively high loss path in the other direction. Considered in terms of the end result achieved, therefore, the device is analogous to a crystal rectifier or diode in that it exhibits a low series impedance in the forward direction and a high series impedance in the reverse direction7 the terms insertion loss and isolation being employed to designate the attenuation of signals applied in the forward and reverse directions, respectively.

Although load isolators may be utilized in several different forms of microwave systems, they have been found to be especially applicable to radar systems for non-reciprocally isolating the magnetron or rnicrowave generator from the remainder of the microwave circuit. More particularly, if the load isolator is conpled to the magnetron and is oriented so that its forward direction is from the magnetron to the associated duplexer or antenna, then substantially all of the output power extracted from the magnetron is transmitted through the isolator on toward the antenna. However, any reflections which would normally be returned to the magnetron are attenuated or absorbed by the load isolator, thereby greatly decreasing the magnetrons sensitivity to load variations and substantially eliminating for practical purposes what is known to the art as long line effect In addition, the utilization of a load isolator permits the degree of coupling between the resonant system of the magnetron and its associated output structure to be increased; consequently, the minimum power output from a system may be increased by employing a load isolator even though the magnetron output signal is attenuated slightly by the insertion loss of the isolator. It is therefore clear that if space and weight considerations permit, it is highly desirable to incorporate load isolators in substantially all radar systems.

In the prior art two different types of load isolators have been developed, the first of which is frequently called a gyrator and is disclosed in U.S. Patent No. 2,644,93O` for Microwave Polarization Rotation Device and Coupling Network, issued July 3, 1953, to C. H. Luhrs et al. In this particular type of prior art load isolator a ferrite wafer is positioned traversely in a cylindrical waveguide to whose ends a pair of rectangular waveguides are attached, the transverse axis of one of the rectangular guides being disposed at an angle of 45 with respect to the transverse axis of the other rectangular waveguide. In addition the isolator includes a magnetic field generator, frequently an electromagnet, for subjecting the ferrite specimen to a magnetic field of predetermined strength.

The mode of operation of load isolators of the type disclosed in the aforementioned patent is described in detail in an article entitled The Microwave Gyrator by C. L. Hogan, published in the January 1952 issue of The Bell System Technical Journal. Briefiy stated, this form of load isolator provides a transmission system which is one-half wavelength longer in one direction than in the other, and more specifically, utilizes the wave rotating properties of ferrite which are analogous to the phenomenon of Faraday rotation in Optics. Accordingly, if the rectangular waveguides are properly oriented relative to each other, energy propagated in one direction is passed while energy transmitted in the other direction is blocked due to the orientation of the terminal rectangular waveguide.

In general load isolators of the above type have been found to have a large number of inherent disadvantages which discourage their incorporation in microwave systems. Firstly, the units require two waveguide transformers for interconnecting the cylindrical waveguide with the two rectangular waveguides. consequently, there is some degree of mismatch which produces undesirable refiections even if the transformers are Well designed. Secondly, the unit is mechanically complex, lengthy and often requires a twist in one of the rectangular waveguides; accordingly the units are relatively expensive. Thirdly, the isolation and insertion loss provided by the unit vary greatly over even a small portion of the microwave Spectrum owing principally to the fact that the angular orientation of the terminal rectangular guides is fixed while the angular rotation provided by the ferrite for a given static field varies with the frequency of the incident energy. Still another important disadvantage of this form of prior art load isolator is that its power handling capacity is very limited owing to the fact that the physical disposition of the ferrite in the waveguide prevents rapid dissipation of the heat generated in the ferrite by the absorption of electrical energy; consequently blowers or a water jacket must be employed With the unit if even a moderate amount of power is to be dissipated therein. Again, the static magnetic field strength for a particular degree of isolation and insertion loss is critically dependent upon frequencies and the temperature of the ferrite; in order to obtain uniform isolation, therefore, an electromagnet servoed from the associated radar system is often required.

In order to overcome these disadvantages another load isolatorhas been developed which employs one or more ferrite specimens asymmetrically mounted within an ordinary rectangular waveguide parallel to the longitudinal axis of the guide. In this form of load isolator the ferrite specimens are preferably rectangular in cross section and are positioned in a static transverse magnetic field provided by a relatively large permanent magnet which contacts the outside periphery of the waveguide walls adjacent the regions whereat the ferrite specimens are affixed to the interior of the waveguide. As in the case of the other prior art load isolator which operates on a principle analogous to Faraday rotation, this newer type of load isolator depends on the fact that the permeability of a ferrite specimen in the presence of a static magnetic field is a non-symmetrical tensor. Consequently, when one or more ferrite specimens are asymmetrically positioned in a waveguide and a static magnetic field is applied transversely to the waveguide, the propagation constant of the modes depends upon the direction of propagation. A more rigorous analysis of this prior art load isolator is set forth on page 816 of the June 1953 issue of the Journal of Applied Physics in a letter to the editor by Kales, et al., entitled A Non-Reciprocal Microwave Component Although the latter vform of prior art load isolator as originally conceived was successful in eliminating many of the disadvantages attendant the original load isolators which employed cylindrical waveguides, it too had several serious disadvantages. Firstly, the permanent magnet required to produce the static magnetic field was relatively large and bulky, thereby severely restricting the configuration of the magnet and the use of the isolator in many applications where spacing of the microwave components was a critical factor. In addition, the utilization of a relatively large permanent magnet increased the weight of the isolator by several pounds as a consequence of which its use in airborne systems was restricted.

As disclosed in copending U.S. patent application Serial No. 486,090 filed on February 4, 1955, now Patent No. 2,776,412, by Arthur Jodeau Sparling for Microwa've Components, the above enumerated disadvantages of this newest form of load isolator have been ameliorated to a large extent by the use of ferrous pole pieces in the sides of the waveguide walls -for interconnecting the ferrite specimen or specimens with the permanent magnet through a low relucta-nce path. This contribution to the art has succeeded in reducing the weight of load isolators by a factor of two or more, and has also materially decreased their cross-sectional size.

Although Sparling's improvement in load isolators has greatly enhanced their utility, there remain 'a number of very serious objections to the incorporation of load isolators in microwave systems. For example, the length of load isolators is very often too large to enable them to be readily incorporated in microwave systems where, as previously stated, the spacing between components and the length of waveguide runs is frequently a critica'l factor and must be held to an absolute minimum. Secondly, even though Sparling's contribution to the art has greatly reduced the weight of load isolators, the weight factor may still be an impedment to the incorporation of a load isolator in a particular system, especially an airborne microwave system. Thirdly, and this disadvantage is again generic to all of the prior art load isolators, the magnet is exposed and great care must be taken to prevent damage thereto and to prevent magnetic materials from partially bdging the air gap thereof.

More specifically, since the isolation provided by an isolator is a relatively critical function of magnet strength, and since any magnet will be degaussed to some extent if it is struck even lightly, an isolator With an unprotected magnet may be rendered useless unless very carefully handled. On the other hand, despite proper handling, the isolator may be rendered temporarily useless in service if a magnetic object is permitted to even partially bridge the gap between the magnet pole pieces, an event which frequently occurs when the magnet is unprotected.

-In order to overcome the above disadvantages the present invention provides load isolators which have both minimal length and weight, and in addition, which provide protection for the permanent magnet employed therein. According to the basic concept of the invention, the load isolators herein disclosed are foreshortened by the elirnination of the conventional waveguide flanges of the prior art and the portions of the waveguide which normally run from the magnet to these fianges, the isolators of the invention including instead a pair of unitary elements which function as electrical connectors and mechanically terminate the isolator waveguide immediately adjacent the ends of the magnet, which function to position and clamp the magnet in a fixed position relative to the ferrite specirnens within the waveguide, and in addition, which serve as mounting members for a magnet-protecting cover. Consequently the invention provides load isolators which are not only lighter and smaller, but in which fewer elements perform more functions with a concomitant reduction in labor and material costs.

In accordance with different embodiments of the invention the end flanges of the load isolators herein disclosed may be utilized to directly fasten the associated magnet at its ends with set screws or the like, or may be slotted to receive one or more clamping bars which in turn engage and clamp the magnet at its outer periphery. In addition, the cover utilized to protect the magnet and the peripheral Shape of the end fianges upon which the cover is mounted may have almost any desired configuration.

It is, therefore, an object of the invention to provide microwave load isolators of 'minimal length and weight wherein at least one of the isolator waveguide fianges serves also as a mounting bracket -for an associated permanent magnet employed to magnetically bias a ferrite specimen within the waveguide.

Another object of the invention is to provide load isolators of minimal length and Weight wherein the waveguide fianges afi'ixed to the ends of the isolator waveguide are also employed for mounting an associated permanent magnet which is utilized to magnetically bias one or more ferrite specimens mounted within the waveguide.

A further object of the invention is to provide microwave load isolators wherein a waveguide section is terminated in flanges at least one of which performs the threefold purposes of serving firstly as a waveguide connector, secondly vas a mounting bracket for an associated Permanent magnet employed to magnetically 'bias one or more ferrite specimens positioned within the waveguide, and thirdly as a cover mounting bracket to which a ma'gnet-protecting cover may be aflixed.

The novel vfeatures which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

PIG. 1 is -an isometric view of one form of -load isolator, according to the invention;

FIG. 2 is a sectional front elevation of the load isolator of FIG. l taken perpendicular to plane 2-2' in FIG. 3;

FIG. 3 is a sectional side elevation of the load isolator of FIG. 1 taken perpendicular to plane 3-3' in FIG. 2;

FIGS. 4 and 5 -are a sectional front elevation and a fragmentary side elevation of a modified form of load isolator, according to the invention, and o FIG. 6 is a sectional front elevation of still another form of load isolator, according to the invention, which illustrates .an alternative method for mounting the associated magnet.

With reference now to the drawings, wherein like or corresponding parts lare designated by the same reference characters throughout the several views, there is shown in FIG. 1V a load isolator, constructed in accordance with the invention, which includes as basic elements a pair of multiple-purpose waveguide flange members 10 and 12 which are utilized as electrical connectors for terminating an associated waveguide section 14 contiguous with the ends of a magnet 16, for mounting magnet 16 in a predetermined position relative to the waveguide, and which 'are further utilized for mounting an associated magnetprotecting cover 18. In addition there is illustrated in this view of the load isolator of the invention an end of an isolator pole piece 20 inset in a wall of the waveguide and the end of a ferrite specimen 22 which is mounted within the waveguide on the pole piece. A further description of these elements, together with a statement of their functions, will be set forth hereinafter.

As illustrated by flange 10 in FIG. l, each of the waveguide flanges is either brazed, soldered or otherwise aflixed to waveguide 14, and includes a plurality of tapped holes 24 which are utilized for aflixing to the isolator mating waveguide flanges, not shown, which terminate waveguide runs from the equipment to which the load isolator is to be connected. It will be noted that flange is in essence a fiat or cover flange so that the waveguide flange which mates therewith when the isolator is incorporated in a microwave system would preferably be a choke flange. It will be recognized from the description set forth hereinafter, however, that flanges 10 and 12 could be constructed as choke flanges without departing from the spirit of the invention, in which instance of course the mating flanges to be connected thereto would be cover flanges.

With reference now to FIGS. 2 and 3, there are shown sectional front and side elevations, respectively, of the load isolator of FIG. 1 illustrating the arrangement of elements therein and the manner in which end flanges 10 and 12 are employed to perform their multiple purposes. Referring now with particularity to both FIGS. 2 and 3, it will be noted that in this specific embodiment of the invention magnet 16 is fixedly positioned in contact with the top of waveguide 14 by one or more set screws 26 which pass through a clamping bar 28, the ends of the clamping bar in turn being held fast within a pair of slots 30 machined in the upper portion of the end flanges. As shown in FIG. 2, the poles of magnet 14 are thus Positioned adjacent ferrite specimen 22 and a second ferrite specimen 23 which is mounted on the opposite wall of waveguide 14, these specimens functioning under the control of the magnetic field applied by magnet 16 to provide the unidirectional characteristics of the load isolator.

lt will also be noted from FIG. 2 that this particular embodiment of the invention also includes a pair of ferrous pole pieces 20 and 21 which are inset in the Walls of waveguide 14 and which function to channel through the ferrite specimens the magnetic flux from magnet 16 by intercoupling the magnet and the specimens through a low reluctance path. As previously set forth, this particular feature of the load isolator is disclosed and claimed in copending U.S. patent application Serial No. 486,090 by Sparling, and functions to decrease materially the weight and cross-sectional area of the isolator separate and apart from the weight-saving and length-reducing contributions of the present invention.

With reference once more to FIG. 3, there is also illustrated one technique which may be utilized for affixing magnet-protecting cover 18 to the waveguide flanges; as shown in this view of the load isolator each of flanges 10 and 12 includes a tapped hole in its periphery for receiving a pair of set screws 32 and 34 which are seated substantially fiush with cover 18. It will be appreciated that these set screws, as well as the set screws 26 which pass through clamping bar 28, may be held in position by applying a suitable cement, such as glyptal, to their threads during the assembly operation.

It will also be recognized that end flanges 10 and 12, together with magnet-protecting cover 18, are preferably constructed of a non-magnetic and lightweight material, such as aluminum for example, to preclude any diversion of the magnetic field and to keep the weight of the device as low as possible. Thus, as shown in FIGURE 2, cover 18 is operative to maintain a substantial distance between the magnet pole pieces and any magnetic material which may contact the external periphery of the load isolator, and in addition, serves to prevent accidental degaussing of the magnet which might otherwise occur through inadvertently perrnitting an object to strike the exterior of the magnet. It should be pointed out, however, that in certain special instances it may be desirable to employ a ferromagnetic cover Well spaced from the magnet in order to prevent stray fields from the isolator from atfecting nearby equipment.

It is to be understood, of course, that the cross-sectional shape of the magnet protecting cover may have any desired configuration, and may be afiixed to the end flanges with fasteners other than set screws. With reference to FIGS. 4 and 5, for example, there is shown a modified form of load isolator wherein magnet protecting cover 18 and the periphery of the end flanges are rectangular in shape. In addition, this modified load isolator embodiment includes a magnet 16 whose configuration is essentially cylindrical and which is held in position relative to waveguide 14 and the associated end flanges by a clamping bar 28 which engages a shoulder at the top of the end flanges.

It should also be recognized that although the load isolators of FIGURES 1 through 5 have their magnets in contact with the upper portion of the waveguide, the isolator may be constructed with its magnet spaced from the top of the guide. For example, there is shown in FIG. 6 a cross-sectional view of still another form of load isolator, according to the invention, wherein magnet 16 is held in place at each end by three set screws which are threaded through the end flanges, as indicated in FIG. 6 by the dotted lines 36, 38 and 40. It will be recognized that the ends of the magnet may be drilled or dimpled in the appropriate points in order to provide seats for the set screws and thereby assure maximum rigidity.

It is clear, of course, that still other techniques may be employed for mounting the isolator magnets adjacent the end flanges. For example, in the load isolators of FIG. 6 three clamping bars of the type disclosed in the embodiments of FIGS. l through 5 could be seated at their ends in suitable slots in the end flanges and could be employed to contact and suspend the magnet at spaced points on its periphery, such as points 42, 44, and 46. It should also be pointed out that although the ends of waveguide 14, as shown throughout the drawings, are located off-center with respect to the 'center of the end flanges, load isolators may be constructed in accordance with the invention wherein the waveguide is centered with respect to the end flanges. However, in those isolators wherein a single magnet is employed to surround only a portion of the waveguide, as in the embodiments of the invention shown and described, minimal length and weight concomitant with good magnet protection are preferably provided by locating the waveguide off-center with respect to the end flanges.

Summarizing briefly the contn'butions of the present invention, it has been shown that the use of multiple purpose end flanges in the load isolators of the invention not only provides a substantial reduction in overall length and weight, but provides the additional function of protecting the magnet notwithstanding the fact that there has been a decrease in the number of elements as contrasted with the load isolators of the prior art. In practice, as a matter of fact, load isolators constructed in accordance with the invention are frequently as much as forty percent shorter and twenty percent lighter than the corresponding structures of the prior art, while operational problems previously caused by accidental degaussing of the magnet or magnetic short circuits are substantially eliminated.

It is to be expressly understood, of course, that the invention may be practiced otherwise than as specifically shown and 'described. For example, for some very specific applications it may be desirable to employ one of the multiple purpose unitary end flanges at only one end of a load isolator, while the other end of the load isolator is termnated in a conventional manner with an additional waveguide run and conventional flange, the opposite end of the magnet being supported by a magnet mounting bracket which encircles but does not terminate the associated waveguide. Accordingly, the spirit and scope of the invention are to -be limited only by the spirit and scope of the appended claims.

This case is a continuation of Serial No. 534,072 filed September 13, 1955, now abandoned.

What is claimed as new is:

1. A microwave load isolator comprising: a short section of waveguide, a ferrite specimen mounted within said waveguide asymmetrically with respect to the central axis of the waveguide, an elongated magnet disposed along the length of the Waveguide, said magnet having a substantially horseshoe shaped configuration in cross-section and circumambiently subscribing an arc about the Waveguide encompassing the central axis thereof and surrounding a greater portion of the periphery of the Waveguide, whereby a greater portion of the cross-section of said waveguide is disposed substantially inside the magnet horseshoe than extends outside thereof, said magnet having first and second ends postioned opposite the ferrite specimen to bias the specimen, a pair of flanges aflixed on the Waveguide and spaced apart from one another by a distance slightly greater than the length of the magnet whereby the opposite sides of the magnet are each contiguous to a different one of the flanges, a pluralty of adjustable screws supported with respect to the fianges and adjusted 15 to bear against said magnet at a plurality of discrete positions to maintain said magnet in a given location With respect to the flanges, and a protective cover completely surrounding the magnet and sides of the waveguide and afl'lxed to the peripheryofthe flanges. v

2. In the isolator of lclaim 1, said screws being threaded` transversely throughsaid flanges to bear against the opposite sides of the magnet.

3. In the isolator of claiml, a clamping bar being supported by and passing betvveen said pair of flanges and said screws being threadedthrOUgh said clamping bar to 10 bear against said magnet.

References ited'in the file of this patent UNITED STATES PATENTS 2,776,412 Sparling Jan. 1, 1957 2,784,378 Yager Mar. 5, 1957 2,834,947 Weisbaum May 13, 1958 

